The fetchart plugin would silently drop unknown sources defined in
config, leading to hard to debug problems.
The plugin now errors when an unknown source is configured, or when no
sources are configured.
In addition, a single string is now a valid value for `sources` to
either enable all sources with an `*` or a single source.
Fixes: #6336
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
